We have been studying factors which control the migration and growth of connective tissue cells. One of these factors, the platelet-derived growth factor (PDGF) appears to be a wound hormone. PDGF is released by platelets during aggregation and is a potent chemoattractant for both smooth muscle cells and fibroblasts. In combination with other factors contained in plasma, PDGF stimulates the growth of these cells. The migratory response of the cells occurs prior to the initiation of DNA synthesis or of cell division. Addition of PDGF to wound models stimulates connective tissue formation. These studies suggest that PDGF might be therapeutically useful in stimulating would repair. Related studies indicated that chemotactic factors may play an important role in the initiation of fibrotic disorders where smooth muscle-like cells appear to the responsible for the altered wound healing. In other studies we have shown that transformed cells lose their ability to respond to PDGF. This loss of responsiveness may be due to production of PDGF like factors by the transformed cells themselves. We find that similar factors are produced by some embryonic cells suggesting that the transformed cells are expressing genes normally expressed only during embryonic development. These PDGF-like molecules produced during embryogenesis may regulate the migration and growth of mesenchymal cells at various stages of development. Production of these factors by tumor cells may be required for the growth and spread of certain tumors.